A head mounted display (“HMD”) is a display device worn on or about the head. HMDs usually incorporate some sort of near-to-eye optical system to emit a light image within a few centimeters of the human eye. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Some HMDs display only a computer generated image (“CGI”), while other types of HMDs are capable of superimposing CGI over a real-world view. This latter type of HMD can serve as the hardware platform for realizing augmented reality. With augmented reality the viewer's image of the world is augmented with an overlaying CGI, also referred to as a heads-up display (“HUD”).
HMDs have numerous practical and leisure applications. Aerospace applications permit a pilot to see vital flight control information without taking their eye off the flight path. Public safety applications include tactical displays of maps and thermal imaging. Other application fields include video games, transportation, and telecommunications. There is certain to be new found practical and leisure applications as the technology evolves; however, many of these applications are limited due to the cost, size, field of view, and efficiency of conventional optical systems used to implemented existing HMDs.
FIG. 1 illustrates a conventional near-to-eye optical system 100 using holographic diffraction gratings 105 disposed on the backside of a waveguide structure 110. Waveguide structure 110 transports image light from an image source 115 around the front of a viewer's face to their eye 120. Since the human eye is typically incapable of focusing on objects placed within five to ten centimeters, this system requires a lens 125 interposed between the waveguide structure 110 and image source 115. Lens 125 creates a image that is virtually displaced further back from the eye by positioning image source 115 inside of the focal point of lens 125. Lens 125 is typically a bulky element.
Optical system 100 uses holographic diffraction gratings 105 disposed on the backside of waveguide structure 110 in place of conventional planar minors to in-couple and out-couple light in and out of waveguide structure 110. Diffraction gratings 105 are single phase diffraction gratings that simply reflect or redirect the light for in and out coupling purposes. These single phase diffraction gratings are inefficient reflectors, since they only reflect higher order diffractions while passing the first order diffraction, which contains the largest portion of energy in an optical wave front. In addition to being poor optical reflectors, the input and output diffraction gratings must be precisely tuned to one another, else the output image will suffer from color separation. Achieving a sufficient match between the input and output gratings 105 requires extreme control over manufacturing tolerances, which is often difficult and costly. Finally, optical system 100 suffers from a limited field of view, which is constrained by the width of waveguide structure 110 which guides the light between the in-coupling and out-coupling diffraction gratings 105. The optical width can be marginally increased by using a high index material to improve field of view, but ultimately the physical dimensions of waveguide structure 110 provide the greater control over field of view. Increasing the width of waveguide structure 110 results in a bulky, uncomfortably heavy and awkward looking eyepiece.